Nucleic acid hybridization studies using cloned retroviral DNA are being used to examine the sites of integration and sequence organization of viral sequences in mammalian cells. One of the goals of these studies is to characterize the endogenous RNA tumor genomes in normal and malignant human tissues. Sequences related to various subclones of the endogenous baboon type C virus have been cloned from a gorilla DNA library. These gorilla viral sequences will be used to probe various human tissues. In addition, the concept of retroviruses as transposable elements between species is being used as a marker for evolutionary rates of mammalian divergence. We have measured precisely the time of interspecies virus transfer and used this to measure the rates of accumulation of nucleotide substitutions in the mammalian lineages that have diverged after the viral transfer occurred. An endogenous virus is being cloned from carnivore cell DNA that is related to the colobus monkey type C virus, and will be used as a probe in carnivore evolutionary studies. A complete phylogenetic tree of carnivore evolution has been derived by molecular hybridization techniques - it differs in many respects from the classical relationships derived by anatomical considerations and morphometric measurements.